nvme: Sanity check completion id

Make sure the completion ID is in the range of [0..num_trackers) since
the values past the end of the act_tr array are never going to be valid
trackers and will lead to pain and suffering if we try to dereference
them to get the tracker or to set the tracker back to NULL as we
complete the I/O.

Sponsored by: Netflix
Reviewed by: mav, chs, chuck
Differential Revision: https://reviews.freebsd.org/D32088